Incremental update leaves the structure of any original document
intact and appends any changes to the end of the file.

Because the original PDF is unchanged, it is possible to revert
back to the state before the changes were applied. Because only
updated objects are written to disk it can result in faster write
times.

However, most importantly, you have to use incremental update if
you wish to preserve any signed signature fields present in the
document. Indeed, after any signature field is signed, you need to
commit changes to file using incremental update.